The present invention relates to a method of producing a ferroelectric memory, in particular, a non-volatile memory using a ferroelectric capacitor.
Recently, a semiconductor memory (ferroelectric memory) using a ferroelectric capacitor has been focused on as a non-volatile memory. A ferroelectric has a characteristic of holding a bi-polar in a voltage application direction even after a voltage is removed (self-polarization). Accordingly, the ferroelectric memory can be applied as a non-volatile memory. The ferroelectric switches polarization thereof at a rate of an order of nano-second. It is also possible to decrease a voltage for switching polarization of the ferroelectric about 2.0 V through optimization of a manufacturing method of a ferroelectric layer. For these reasons, as compared with a flash memory or an EEPROM (Electrically Erasable Programmable Read-Only Memory), the ferroelectric memory has significant advantages in terms of a re-writing speed and an operation voltage. Further, it is possible to re-write data in the ferroelectric memory more than 1012 times. The ferroelectric capacitor has been commercially available as an RAM (Random Access Memory).
In the ferroelectric memory, it is necessary to reduce deterioration due to imprint for obtaining long term reliability. The imprint is a phenomenon in which, after data is stored in the ferroelectric memory and is held therein for a long period of time, a characteristic of holding data opposite to the stored data is deteriorated. When certain data is held in the ferroelectric memory, floating electron charges in the ferroelectric capacitor are re-distributed to form an internal electric field aligned in a direction of polarization, thereby causing the imprint.
The imprint is associated with and influenced by floating electron charges in a ferroelectric layer of the ferroelectric capacitor. Further, the imprint is influenced by deformation (damage) of crystal cause by hydrogen entering the ferroelectric layer during a manufacturing process after the ferroelectric capacitor is formed. Accordingly, it is necessary to provide a step for preventing the damage in the ferroelectric layer in the manufacturing process after the ferroelectric capacitor is formed.
For example, a step has been performed in which a cover layer such as Al2O3 is formed right after a ferroelectric capacitor is formed, so that hydrogen does not reach the ferroelectric layer, thereby preventing damage. In such a step, it is necessary to form the cover layer in the additional step, thereby making a manufacturing process complex.
As another example, restoration annealing has been performed for improving ferroelectric characteristic of a ferroelectric capacitor (see Patent References 1 to 4). After the ferroelectric capacitor is formed, a contact hole is formed through etching for contacting an electrode of the ferroelectric capacitor. During the etching, the ferroelectric capacitor may be damaged by hydrogen entering when an oxide layer is formed as an interlayer insulating layer. Accordingly, in Patent Reference 1, the restoration annealing is performed under oxygen for restoring the damage. Such restoration annealing may be performed under ozone (Patent Reference 2), nitrogen (Patent Reference 3), or atmosphere (Patent Reference 4). Patent Reference 1; Japanese Patent Publication (Kokai) No. 10-247724
Patent Reference 2; Japanese Patent Publication (Kokai) No. 06-13565
Patent Reference 3; Japanese Patent Publication (Kokai) No. 08-8409
Patent Reference 3; Japanese Patent Publication (Kokai) No. 2003-324186
When the restoration annealing is performed in the conventional method, crystallinity of a damaged portion is restored, so that the damaged portion can function as a ferroelectric. However, it is difficult to completely restore the damaged portion, and the damaged portion still includes floating electron charges. When an electric field is applied between electrodes of the ferroelectric capacitor to align polarization, the floating electron charges move toward the electrodes, thereby creating an internal electrical field and shifting hysteresis.
In view of the problems described above, an object of the present invention is to provide a method of producing a ferroelectric memory having a small shift of hysteresis even when a ferroelectric capacitor contains floating electron charges.
Further objects and advantages of the invention will be apparent from the following description of the invention.